1.The reason why two fold higher urea concentrations in the haemolymph in the silkworm larvae reared on an artificial diet result than those reared on fresh mulberry leaves was investigated by measuring arginase activity in the fat body tissue.2.Assay conditions for arginase in the fat body was studied using the tissues from the fifth instar larvae.3.Fat body arginase activity was measured from the beginning of the fifth instar larvae until the pupae at one day before adult emergence using insects reared on an artigicial diet and on fresh mulberry leaves.Silkworms reared on either diet showed similar activity changes with a peak at the middle of the fifth instar larvae. Silkworms reared on an artificial diet showed two fold higher activity per individual insect basis than those reared on fresh mulberry leaves.5.Argianse activity per mg fat body protein showed similar changes in the level both in the larvae reared on an artificial diet and on fresh mulberry leaves. Protein content per individual insect basis was higher in the larvae reared on an artificial diet. These results suggest that proteins synthesized in the fat body in response to intake of an artificial diet involves arginase in it.6.Subcellular localization studies indicated that arginase of silkworm larval fat body localized in mitochondrial matrix. This result was in contrast to the widespread belief of arginase localization in cells. Ammonotelic animal arginse in the mitochondrial matrix. Uricotelic animal arginase in mitochondrial innnermembrane. Ureotelic animal arginase in cytosol. Further studies should unveil the mystery.